The present invention relates to an equatorial instrument for an astronomical telescope, and particularly, to an electronic polar scope, a polar axis calibration system and a polar axis calibration control device for the equatorial instrument, as well as an astronomical telescope comprising any of the aforesaid devices.
The Earth rotates with its rotation axis pointing to the celestial pole, that is, the north celestial pole in the Northern Hemisphere, or the south celestial pole in the Southern Hemisphere. Due to the rotation of the Earth, a variety of celestial bodies are watched to be rotating around the celestial pole. However, the astronomical shooting takes a long time for exposure, if the asters are in move, the pictures thus obtained would be an aster track rather than an aster spot. In order to shoot an aster spot, it is needed to drive the telescope to make synchronous rotation while tracing according to the angular velocity of the Earth rotation. The equatorial instrument for the astronomical telescope implements the rotation and tracing by means of its right ascension axis (also called “polar axis”). To implement the rotation and tracing, the polar axis shall accurately point to the north celestial pole or the south celestial pole. So when the equatorial instrument is set up, the first step is to calibrate the polar axis.
The polar axis calibration is usually dependent on the polar scope, and now, almost all of the equatorial instruments are equipped with polar scope. The polar scope is also called polar axis telescope. Existing polar scopes are all optical polar scopes. The optical polar scope is mounted on the right ascension axis of the equatorial instrument, and its optical axis is required to be parallel with the right ascension axis of the equatorial instrument, thus the center of viewing field of the optical polar scope represents the pointing direction of the right ascension axis.
The method for calibrating the polar axis of the equatorial instrument by means of the existing optical polar scope is substantially as follows:
One polar axis reticle is installed in front of the ocular of the polar scope, and the polar axis reticle is marked with the optical center of the polar scope and the position of the North Star. The North Star doesn't exactly locate at the north celestial pole, but goes away from the north celestial pole by a certain distance. Therefore, in the actual adjustment of the polar axis, it is not simply to place the North Star at the optical center of the telescope, but to place the North Star in the polar axis reticle at the position of the North Star. Due to the rotation and revolution of the Earth, the orientation of the North Star relative to the North Pole varies at different dates of the year and different times of the day as well as at difference locations on the Earth in view of the horizon system of coordinates. Moreover, its position would revolve around the north celestial pole by one revolution everyday. Therefore, it is required for existing techniques to calculate the orientation of the North Star relative to the north celestial pole according to the recent time, date and geographical location; then adjust the position of the North Star in the reticle into this orientation; then adjust the horizontal axis and elevation angle of the equatorial instrument so as to place the actual North Star at the position of the North Star in the reticle.
The above prior art for polar axis calibration has two characteristics: 1. the alignment of the celestial pole position is conducted indirectly through the alignment of the position of the North Star; and 2. the optical axis of the polar scope is required to be parallel with the polar axis of the equatorial instrument.
The above techniques for polar axis calibration have the following deficiencies:
Firstly, a pure optical method is employed, whose accuracy is not very high. If a higher accuracy is intended to obtain, a higher magnification for the polar scope is required. However, when the magnification is too great, the viewing field would become very small. Finding the North Star will be very difficult. At present, the field of view of the polar scope of all kinds of equatorial instruments is not great, thus it is not easy to find the aster now. Therefore, the accuracy and the field of view are contradictory. The existent polar scopes have negative impact upon the two aspects when balancing the two.
Secondly, due to the fact that the celestial pole position is indirectly aligned by means of the alignment of the position of the North Star, the polar axis calibration accuracy is subjected to influence of deviation occurred when determining the position of the North Star with respect to the celestial pole;
Thirdly, it is required to ensure the optical axis of the polar scope be parallel with the right ascension axis of the equatorial instrument, which is also very difficult. Moreover, it is difficult to achieve a very high parallelism, resulting in the so-called “taper error”. At the same time, it also makes a very high demand for the assembly.
Fourthly, due to the fact that the reference coordinate system is a horizon system of coordinates, the base of the equatorial instrument has to be horizontal so as to ensure that the calculated position on the reticle is made with respect to the horizon system of coordinates. The obtaining of a very precise horizontal is very difficult. The current equatorial instruments each use the spirit bubble to observe the horizontal. Such a manner itself presents a large error, and has difficulty in achieving a horizontal adjustment of high precision. Accordingly, the adjustment of the polar axis also has difficulty in achieving a high precision.
The Chinese patent application CN201210106129.1 discloses a polar axis assisted adjustment system and method for the equatorial astronomical telescope, however, such a system and method still require that the optical axis of the polar scope is parallel with the polar axis of the equatorial instrument, and the position of the North Star is adjusted so as to adjust the celestial pole position, its essence is consistent with the above-mentioned existent method, and differs only in that an image collecting device is used to replace the human eye and a locating device is used to obtain the position where the North Star should be in the image of the polar scope (i.e., the information of the second position of the North Star as described in CN201210106129.1).